swap ordering so PACKER_PLUGIN_PATH gets precedence

config.go

@@ -123,22 +123,6 @@ func (c *config) Discover() error {
 		return nil
 	}
 
-	// First, check whether there is a custom Plugin directory defined. This gets
-	// absolute preference.
-	if packerPluginPath := os.Getenv("PACKER_PLUGIN_PATH"); packerPluginPath != "" {
-		sep := ":"
-		if runtime.GOOS == "windows" {
-			// on windows, PATH is semicolon-separated
-			sep = ";"
-		}
-		plugPaths := strings.Split(packerPluginPath, sep)
-		for _, plugPath := range plugPaths {
-			if err := c.discoverExternalComponents(plugPath); err != nil {
-				return err
-			}
-		}
-	}
-
 	// Next, look in the same directory as the executable.
 	exePath, err := osext.Executable()
 	if err != nil {
@@ -164,6 +148,22 @@ func (c *config) Discover() error {
 		return err
 	}
 
+	// Check whether there is a custom Plugin directory defined. This gets
+	// absolute preference.
+	if packerPluginPath := os.Getenv("PACKER_PLUGIN_PATH"); packerPluginPath != "" {
+		sep := ":"
+		if runtime.GOOS == "windows" {
+			// on windows, PATH is semicolon-separated
+			sep = ";"
+		}
+		plugPaths := strings.Split(packerPluginPath, sep)
+		for _, plugPath := range plugPaths {
+			if err := c.discoverExternalComponents(plugPath); err != nil {
+				return err
+			}
+		}
+	}
+
 	// Finally, try to use an internal plugin. Note that this will not override
 	// any previously-loaded plugins.
 	if err := c.discoverInternalComponents(); err != nil {